.\" -*- nroff -*-
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Comm_join 3 "Jun 10, 2020" "4.0.4" "Open MPI"

.SH NAME
\fBMPI_Comm_join\fP \- Establishes communication between MPI jobs

.SH SYNTAX
.ft R

.SH C Syntax
.nf
#include <mpi.h>
int MPI_Comm_join(int \fIfd\fP, MPI_Comm *\fIintercomm\fP)

.fi
.SH INPUT PARAMETER
.ft R
.TP 1i
fd
socket file descriptor (socket).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
intercomm
Intercommunicator between processes (handle).
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Comm_join creates an intercommunicator from the union of two MPI
processes that are connected by a socket. \fIfd\fP is a file
descriptor representing a socket of type SOCK_STREAM (a two-way
reliable byte-stream connection). Nonblocking I/O and asynchronous
notification via SIGIO must not be enabled for the socket. The socket
must be in a connected state, and must be quiescent when MPI_Comm_join
is called.
.sp
MPI_Comm_join must be called by the process at each end of the
socket. It does not return until both processes have called
MPI_Comm_join.

.SH NOTES
.ft R
There are no MPI library calls for opening and manipulating a socket.
The socket \fIfd\fP can be opened using standard socket API calls.
MPI uses the socket to bootstrap creation of the intercommunicator,
and for nothing else. Upon return, the file descriptor will be open
and quiescent.
.sp
In a multithreaded process, the application must ensure that other
threads do not access the socket while one is in the midst of
calling MPI_Comm_join.
.sp
The returned communicator will contain the two processes connected by
the socket, and may be used to establish MPI communication with
additional processes, through the usual MPI communicator-creation
mechanisms.

.SH ERRORS
.ft R
Almost all MPI routines return an error value; C routines as
the value of the function and Fortran routines in the last argument. C++
functions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI::Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
may be used to cause error values to be returned. Note that MPI does not
guarantee that an MPI program can continue past an error.
.sp
See the MPI man page for a full list of MPI error codes.

.SH SEE ALSO
.ft R
.nf
socket(3SOCKET)
MPI_Comm_create
MPI_Comm_group

